The technology of organic light emitting diodes (OLEDs) is undergoing rapid development. OLEDs originally utilized the electroluminescence produced from electrically excited molecules that emitted light from their singlet states. Such radiative emission from a singlet excited state is referred to as fluorescence. More recent work has demonstrated that higher power efficiency OLEDs can be made using molecules that emit light from their triplet state, defined as phosphorescence.
Such electrophosphorescence makes it possible for phosphorescent OLEDs to have substantially higher quantum efficiencies than are possible for OLEDs that only produce fluorescence. This is based on the understanding that the excitons created in an OLED are produced, according to simple statistical arguments as well as experimental measurements, approximately 75% as triplet excitons and 25% as singlet excitons. The triplet excitons more readily transfer their energy to triplet excited states that can produce phosphorescence whereas the singlet excitons typically transfer their energy to singlet excited states that can produce fluorescence. Since the lowest emissive singlet excited state of an organic molecule is typically at a slightly higher energy than the lowest triplet excited state, the singlet excited state may relax, by an intersystem crossing process, to the emissive triplet excited state. This means that all the exciton excitation energy may be converted into triplet state excitation energy, which then becomes available as phosphorescent emission. Thus, electrophosphorescent OLEDs have a theoretical quantum efficiency of 100%, since all the exciton excitation energy can become available as electrophosphorescence.
As a consequence, since the discovery that phosphorescent materials could be used in an OLED, Baldo et al., “Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices”, Nature, vol. 395, 151-154, 1998, there is now much interest in finding more efficient electrophosphorescent materials.
Typically phosphorescent emission from organic molecules is less common than fluorescent emission. However, phosphorescence can be observed from organic molecules under an appropriate set of conditions. Organic molecules coordinated to lanthanide elements often phosphoresce from excited states localized on the lanthanide metal. The europium diketonate complexes illustrate one group of these types of species. Organic phosphorescence is also often observed in molecules containing heteroatoms with unshared pairs of electrons at very low temperatures. Benzophenone and 2,2′-bipyridine are such molecules. Phosphorescence can be enhanced over fluorescence by confining, preferably through bonding, the organic molecule in close proximity to an atom of high atomic number. This phenomenon, called the heavy atom effect, is created by a mechanism known as spin-orbit coupling. A related phosphorescent transition is a metal-to-ligand charge transfer (MLCT) that is observed in molecules such as tris(2-phenylpyridine)iridium(III).
However, molecules that phosphoresce from MLCT states typically emit light that is of lower energy than that observed from the unbound organic ligand. This lowering of emission energy makes it difficult to develop organic molecules that phosphoresce in the technologically useful blue and green colors of the visible spectrum where the unperturbed phosphorescence typically occurs.
It would be desirable if more efficient electrophosphorescent materials could be found, particularly materials that produce their emission in the blue region of the spectrum.
The realization of highly efficient blue, green and red electrophosphorescence is a requirement for portable full color displays and white lighting applications with low power consumption. Recently, high-efficiency green and red organic electrophosphorescent devices have been demonstrated which harvest both singlet and triplet excitons, leading to internal quantum efficiencies (ηint) approaching 100%. See Baldo, M. A., O'Brien, D. F., You, Y., Shoustikov, A., Sibley, S., Thompson, M. E., and Forrest, S. R., Nature (London), 395, 151-154 (1998); Baldo, M. A., Lamansky, S., Burrows, P. E., Thompson, M. E., and Forrest, S. R., Appl. Phys. Lett., 75, 4-6 (1999); Adachi, C., Baldo, M. A., and Forrest, S. R., App. Phys. Lett., 77, 904-906, (2000); Adachi, C., Lamansky, S., Baldo, M. A., Kwong, R. C., Thompson, M. E., and Forrest, S. R., App. Phys. Lett., 78, 1622-1624 (2001); and Adachi, C., Baldo, M. A., Thompson, M. E., and Forrest, S. R., Bull. Am. Phys. Soc., 46, 863 (2001). Using a green phosphor, fac tris(2-phenylpyridine)iridium (Ir(ppy)3), in particular, an external quantum efficiency (ηext) of (17.6±0.5)% corresponding to an internal quantum efficiency of >85%, was realized using a wide energy gap host material, 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole (TAZ). See Adachi, C., Baldo, M. A., Thompson, M. E., and Forrest, S. R., Bull. Am. Phys. Soc., 46, 863 (2001). Most recently, high-efficiency (ηext=(7.0±0.5)%) red electrophosphorescence was demonstrated employing bis(2-(2′-benzo[4,5-a]thienyl)pyridinato-N,C3) iridium (acetylacetonate) [Btp2Ir(acac)]. See Adachi, C., Lamansky, S., Baldo, M. A., Kwong, R. C., Thompson, M. E., and Forrest, S. R., App. Phys. Lett., 78, 1622-1624 (2001).
In each of these latter cases, high efficiencies are obtained by energy transfer from both the host singlet and triplet states to the phosphor triplet, or via direct trapping of charge on the phosphor, thereby harvesting up to 100% of the excited states. This is a significant improvement over what can be expected using fluorescence in either small molecule or polymer organic light emitting devices (OLEDs). See Baldo, M. A., O'Brien, D. F., Thompson, M. E., and Forrest, S. R., Phys. Rev., B 60, 14422-14428 (1999); Friend, R. H., Gymer, R. W., Holmes, A. B., Burroughes, J. H., Marks, R. N., Taliani, C., Bradley, D. D. C., Dos Santos, D. A., Bredas, J. L., Logdlund, M., Salaneck, W. R., Nature (London), 397, 121-128 (1999); and Cao, Y, Parker, I. D., Yu, G., Zhang, C., and Heeger, A. J., Nature (London), 397, 414-417 (1999). In either case, these transfers entail a resonant, exothermic process. As the triplet energy of the phosphor increases, it becomes less likely to find an appropriate host with a suitably high energy triplet state. See Baldo, M. A., and Forrest, S. R., Phys. Rev. B 62,10958-10966 (2000). The very large excitonic energies required of the host also suggest that this material layer may not have appropriate energy level alignments with other materials used in an OLED structure, hence resulting in a further reduction in efficiency. To eliminate this competition between the conductive and energy transfer properties of the host, a route to efficient blue electrophosphorescence may involve the endothermic energy transfer from a near resonant excited state of the host to the higher triplet energy of the phosphor. See Baldo, M. A., and Forrest, S. R., Phys. Rev. B 62,10958-10966 (2000); Ford, W. E., Rodgers, M. A. J., J. Phys. Chem., 96, 2917-2920 (1992); and Harriman, A.; Hissler, M.; Khatyr, A.; Ziessel, R. Chem. Commun., 735-736 (1999). Provided that the energy required in the transfer is not significantly greater than the thermal energy, this process can be very efficient.
Organic light emitting devices (OLEDs), which make use of thin film materials that emit light when excited by electric current, are expected to become an increasingly popular form of flat panel display technology. This is because OLEDs have a wide variety of potential applications, including cellphones, personal digital assistants (PDAs), computer displays, informational displays in vehicles, television monitors, as well as light sources for general illumination. Due to their bright colors, wide viewing angle, compatibility with full motion video, broad temperature ranges, thin and conformable form factor, low power requirements and the potential for low cost manufacturing processes, OLEDs are seen as a future replacement technology for cathode ray tubes (CRTs) and liquid crystal displays (LCDs), which currently dominate the growing $40 billion annual electronic display market. Due to their high luminous efficiencies, electrophosphorescent OLEDs are seen as having the potential to replace incandescent, and perhaps even fluorescent, lamps for certain types of applications.
Light emission from OLEDs is typically via fluorescence or phosphorescence. As used herein, the term “phosphorescence” refers to emission from a triplet excited state of an organic molecule and the term fluorescence refers to emission from a singlet excited state of an organic molecule.
Successful utilization of phosphorescence holds enormous promise for organic electroluminescent devices. For example, an advantage of phosphorescence is that all excitons (formed by the recombination of holes and electrons in an EL), which are formed either as a singlet or triplet excited state, may participate in luminescence. This is because the lowest singlet excited state of an organic molecule is typically at a slightly higher energy than the lowest triplet excited state. This means that, for typical phosphorescent organometallic compounds, the lowest singlet excited state may rapidly decay to the lowest triplet excited state from which the phosphorescence is produced. In contrast, only a small percentage (about 25%) of excitons in fluorescent devices are capable of producing the fluorescent luminescence that is obtained from a singlet excited state. The remaining excitons in a fluorescent device, which are produced in the lowest triplet excited state of an organic molecule, are typically not capable of being converted into the energetically unfavorable higher singlet excited states from which the fluorescence is produced. This energy, thus, becomes lost to radiationless decay processes that heat-up the device.